In neurosurgery, a fluorophore such as indocyanine green, ICG, is used to provide information related to blood flow in arteries and veins in particular of the brain. The fluorophore is injected intravenously and an NIR fluorescence imaging system, which may be part of the medical observation device, provides continuous imaging of the fluorophore's fluorescence. As the fluorophore follows the blood flow, it passes sequentially through different blood compartments, first the arteries, then the capillaries, and finally the veins. If an observation area of the object is illuminated using fluorescence excitation wavelengths which trigger the fluorescence of the fluorophore in the fluorescence emission wavelengths, the passing of the fluorophore can be observed: The fluorescence intensity in the fluorescence emission wavelengths starts to increase when the fluorophore reaches the observation area. The fluorescence intensity then increases until a maximum is reached. Thereafter, fluorescence intensity decreases due to a washout of the fluorophore and/or chemical reactions, which degrade the fluorophore.
The standard way to evaluate the blood flow using fluorescence imaging is to record a video which is replayed again and again by an observer, such as a surgeon, to observe the evolvement of fluorescence intensities in different parts of the image. Each of the three blood compartments, i.e. arteries, capillaries and veins, exhibits a maximal fluorescence intensity at a different time after the fluorophore injection. Areas without any blood flow, such as clipped vessels, do not exhibit any fluorescence at all. Thus, by watching a video sequence of the fluorescence development, an experienced observer may be able to identify the type of tissue in a specific region. This process requires experience and even for an experienced user, the identification is not reliable.
Therefore, analysis methods have been developed to automatically and reliably identify the various blood compartments. One of the approaches uses a curve-parameter extraction. This method is applied when all time frames of a video sequence are available, i.e. when the fluorescence has died off and there is no or almost no fluorophore anymore in the object. In this method, a time curve of the fluorescence intensity at a particular location is computed using all available frames. Each time curve for each pixel is analyzed to produce one or more parameters which characterize the tissue wrapped onto the respective pixel. Such parameters may be for example the arrival time of fluorescence at a certain pixel, i.e. the point in time when the fluorescence intensity at a pixel exceeds a given threshold for the first time. Another characteristic may be the peak time, i.e. the point in time when the fluorescence intensity at a pixel reaches its maximum. Other parameters such as wash-out time, maximum curve slope and curve integral can also be calculated from each curve.
Using a parameter such as the arrival time and the peak time allows a rough identification of the different blood compartments. A pseudo color image may be generated which assigns a color to a pixel depending on the arrival time. Different blood compartments exhibit different arrival times and thus can be easily distinguished using this curve parameter. For example, arrival time will be short for large arteries, as the fluorophore will reach them first. In contrast, arrival time will be later for large veins which gather blood from a large area. The visualization of the arrival time in a color-coded manner provides a still image which combines the information on the tissue gathered from the time series of frames.
Despite providing great advantages, the curve-parameter extraction method also exhibits some shortcomings. For example, it is required that the whole duration of the dynamic phenomenon is covered. The time series has to show the rise, peak, and decline of fluorescence. This means that the surgeon needs to wait for a few minutes until the analysis can be started and visualized. Moreover, several superficial tissue layers which belong to different blood compartments may be located one above the other and then lead to superimposed fluorescence time curves. In such a situation, the parameters extracted by curve-parameter extraction have a high likelihood of not bearing any physical meaning.